Rounding operation circuit

ABSTRACT

A rounding operation circuit for arithmetic logic unit of a signal processor provided for counting fractions over 1/2 as one and disregarding the rest for the positive and negative number, which comprises a decoder circuit having an (n+1)-long input to which a first input signal represented by two&#39;s complement and a second n-bit long input signal for specifying the rounded position of the first signal are entered, when the first input signal is positive, a signal in which the bit at the rounded position is &#34;1&#34; and the rest is &#34;0&#34; is emitted based on the second input signal and, when the first input signal is negative, a signal in which the bits less significant than the bit at the rounded position are all &#34;1&#34; and the rest is &#34;0&#34; is emitted; arithmetic logic unit for adding the output signal of this decoder circuit and the first input signal; and a rounding circuit for counting 1 and cutting away 0 positively and negatively symmetrically to any rounding position, to allow a fast and accurate rounding operation.

BACKGROUND OF THE INVENTION

The present invention relates to a rounding operation circuit used in an image signal processor.

According to the amendment of the recommendation H.261 of the CCITT p×64 bits/sec. television phone coding system, chances are that a system for DCT (discrete cosine transform) coding between motion compensating frames will be adopted. In this recommendation, an allowable range of the inverted DCT operating error is defined in order to suppress the mismatch of the coded image attended with transmission and reception. These are based on the content of "CCITT SGXV WP XV/1 Specialist Group on Coding for Visual Telephony, Doc, #584, (1988-11)".

In the reverse DCT operation, there is included a so-called rounding operation which cuts off the most significant 16 bits after 1 is added to, for example, the fifteenth bit of 31 bits as counted from below. As this rounding method, various means are conceivable, but it is reported that the foregoing reverse DCT operating error becomes minimal by carrying out the rounding operation symmetrically relative to the positive and negative (that is, rounding to the hearest whole number for absolute value, and cutting away zero and including one for binary number). For example, when the position of the decimal point lies at the eighth bit and the two's complement's 16-bit long signal, if it is positive, then "00000000, 10000000" (=0.5) is added and, if it is negative, then "00000000 01111111" (=0.4961) is added and the numbers lying at the places of the resulting number which are lower than the decimal point are cut away so that they can be rounded. These processes are proposed in a document 1, "Reverse DCT Operation in Motion Picture Processing VISP-LSI" Mochizuki et al, 1990, Telecommunication Association, National Conference Proceedings".

Next, a description is made with reference to the "RND" instruction of the chapter "Assembly Language Instructions" of the document 2 titled "Texas Instrument Third-Generation TMS320 User's Guide 1988, 9".

The instruction format is designated as "RND <src><dst>". Upon this specification, the result of the rounding operation of the source operand (,src.) is stored into a destination register (<dst>). In this case, the figure is rounded so that it becomes the nearest number of the single precision floating decimal point (that is, the eighth digit below the decimal point of the integer). In the case of the right middle value (that is, x. xxxxxxx5, where x denotes the integer value from 0 up to 9), the rounding operation is carried out in the positive direction.

These processes are specifically described hereinafter.

    ______________________________________                                         <src> = 07333C16EEFh = 1.79755599e + 02                                        <dst> = 0h                                                                     After the execution of the instruction:                                        <src> = 0733C16EEFh = 1.79755599e + 02                                         <dst> = 0733C16F00h = 1.79755600e + 02                                         ______________________________________                                    

As shown above, the rounding operation is achieved by representing the hexadecimal number by the decimal system.

Next, description is made with reference to the "RND" instruction of the chapter "Instruction Set Details" included within a third document "DSP56000 Digital Signal Processor User's Manual 1986".

The instruction format is designated as "RND<dst>". Upon this designation, the result of the rounding operation of the designation register <dst> is stored into the destination register <dst> as it is. The rounding operation of this processor is achieved by adding a constant (hereinafter referred to as a "rounding constant") to the bit to be rounded. In this case, the rounding constant is determined by the value of the scaling bit of the status register. Next, a correspondence between the place of this example where the rounding operation is carried out and the added value is shown in Table 1 below:

                  TABLE 1                                                          ______________________________________                                                                 Round-                                                 Status Status           ing                                                    Register                                                                              Register Scaling Posi-  Rounding Constant                               1      2        Mode    tion   [55 . . . 24, 23, 22 . . .                      ______________________________________                                                                        0]                                              0      0        no      23     0 . . . 0                                                                             1 10 . . . 0                                             scal-                                                                          ing                                                            0      1        scale   24     0 . . . 1                                                                             0 00 . . . 0                                             down                                                           1      0        scale   22     0 . . . 0                                                                             0 1 . . . 0                                              up                                                             ______________________________________                                    

According to the RND instruction of the document 2 processor, which is the aforementioned prior art, since a negative number "x, xxxxxxx5" is rounded in the positive direction, it is not possible to round the absolute number by counting fractions over 1/2 as one and cutting away the rest, which in turn makes it impossible to round at the desired bit position. Further, also in the document 3, since the negative number is rounded, it is impossible to round by counting its fractions over 1/2 as one and cutting away the rest leading to the same result.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to remove such a drawback and to provide a rounding operation circuit which allows the fractions over 1/2 of the absolute value to be counted as one and its rest to be cut away.

According to the present invention, there is provided a rounding operation circuit for performing a rounding operation on a first input signal represented in a two's complement form, comprising decoder means for receiving a second input signal designating a rounding position of said first input signal and for outputting a decoded signal in which a bit corresponding to said rounding position takes logic "1" and remaining bits take logic "0" when said first input signal is positive and in which a bit or bits less significant that said bit corresponding to said rounding position takes logic "1" and remaining bit or bits take logic "0" when said first input signal is negative; operational means for performing an adding operation on said first input signal and said decoded signal and for outputting a resultant signal; and rounding-down means performing such a rounding-down operation on said resultant signal that bits of said resultant signal less significant than said rounding position take logic "0".

Furthermore, in a preferred embodiment of the present invention, the second input signal for specifying the rounding position of the first input signal comprises four bits, or the value for specifying the rounding position comprises 16 bits having an immediate value.

In other words, since the rounding operation circuit of the present invention allows the symmetrical rounding operation intended for both the positive and the negative values to be achieved with a single instruction, it has an advantage that a faster and more accurate operation can be achieved than- when the operation is achieved using the conventional arithmetic logic means.

The present invention will be clearly understood from the following detailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the rounding operation circuit embodying the present invention; and

FIG. 2 is a block diagram of a rounding-down circuit shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a first embodiment of the present invention, in which 1 denotes a first 16-bit long input signal, 2 a second 4-bit long input signal, 3 a decoder circuit into which 5 bits are entered and from which 16 bits are emitted, 4 a 16 bit long output signal of the decoder circuit, 5 a most significant bit of the first input signal, 6 an arithmetic logical operation unit or adder, 7 the output of the adder, 8 a rounding-down circuit with the second 4-bit long input signal as the input. In FIG. 2 is illustrated a circuit diagram showing an arrangement of the rounding-down circuit 8 which comprises a decoder 11 and a plurality of, sixteen in this case, AND gates 12. The following Table 2 is a truth table of the decoder 3.

The second input signal bit at the left column of the table 2 represents the bit arrangement from 0 to 15 which are added with the positive sign "0" while the second input signal bit at the right column represents the bit arrangement of the complement which is added with the negative sign "1". For the output of the decoder 3, when the input signal is positive, the rounding bit position is "1" and the rest is "0" while when the input signal is negative, the bits less significant than the bit at the rounding position are "1" and the rest is "0".

                  TABLE 2                                                          ______________________________________                                         Second                Second                                                   Input Decoder         Input   Decoder                                          Signal                                                                               Output          Signal  Output                                           ______________________________________                                         00000 0000000000000001                                                                               10000   0000000000000000                                 00001 0000000000000010                                                                               10010   0000000000000011                                 00010 0000000000000100                                                                               10011   0000000000000111                                 00011 0000000000001000                                                                               10100   0000000000001111                                 00100 0000000000010000                                                                               10101   0000000000011111                                 00101 0000000000100000                                                                               10110   0000000000111111                                 00110 0000000001000000                                                                               10111   0000000001111111                                 00111 0000000010000000                                                                               11000   0000000011111111                                 01000 0000000100000000                                                                               11001   0000000111111111                                 01001 0000001000000000                                                                               11010   0000001111111111                                 01010 0000010000000000                                                                               11011   0000011111111111                                 01011 0000100000000000                                                                               11100   0000111111111111                                 01100 0001000000000000                                                                               11101   0001111111111111                                 01101 0010000000000000                                                                               11110   0011111111111111                                 01110 0100000000000000                                                                               11111   0111111111111111                                 01111 1000000000000000                                                         ______________________________________                                    

Referring now to the operation of the rounding operation circuit according to the present invention, the second input signal 2 having an information regarding the position of the rounded bit of the input signal 1 is entered to a decoder circuit 3, which decodes the input signals 2 and the most significant bit 5 of the first input signal 1 according to the truth table 2, thereby to generate an output signal 4. The output signal 4 and the first input signal 1 are added to each other by means of the arithmetic logic means 6 to generate an output signal 7. In accordance with the input signal 2, the less significant bits than the rounding position in accordance with the input signal 2 are all turned to 0 by means of the rounding-down circuit 8 (FIG. 2), and the result is emitted as the output signal 9. Incidentally, the truth table of the decoder circuit 11 of FIG. 2 is represented in the following table 3.

                  TABLE 3                                                          ______________________________________                                         Second                Second                                                   Input Decoder         Input   Decoder                                          Signal                                                                               Output          Signal  Output                                           ______________________________________                                         0000  1111111111111110                                                                               1000    1111111000000000                                 0001  1111111111111100                                                                               1001    1111110000000000                                 0010  1111111111111000                                                                               1010    1111100000000000                                 0011  1111111111110000                                                                               1011    1111000000000000                                 0100  1111111111100000                                                                               1100    1110000000000000                                 0101  1111111111000000                                                                               1101    1100000000000000                                 0110  1111111110000000                                                                               1110    1000000000000000                                 0111  1111111100000000                                                                               1111    0000000000000000                                 ______________________________________                                    

Alternatively, according to a second embodiment of the present invention, different from the first embodiment, a second 16-bit long input signal 2a and a decoder circuit 3a comprise a 16-bit input and a 16-bit output. In this case, the truth table of the decoder 3a is shown in Table 4 in which the value specifying the rounding position has an immediate value.

                  TABLE 4                                                          ______________________________________                                         Second Input Signal                                                                            Decoder Output                                                 ______________________________________                                         00000000000000001                                                                              0000000000000001                                               00000000000000010                                                                              0000000000000010                                               00000000000000100                                                                              0000000000000100                                               00000000000001000                                                                              0000000000001000                                               00000000000010000                                                                              0000000000010000                                               00000000000100000                                                                              0000000000100000                                               00000000001000000                                                                              0000000001000000                                               00000000010000000                                                                              0000000010000000                                               00000000100000000                                                                              0000000100000000                                               00000001000000000                                                                              0000001000000000                                               00000010000000000                                                                              0000010000000000                                               00000100000000000                                                                              0000100000000000                                               00001000000000000                                                                              0001000000000000                                               00010000000000000                                                                              0010000000000000                                               00100000000000000                                                                              0100000000000000                                               01000000000000000                                                                              1000000000000000                                               10000000000000001                                                                              0000000000000000                                               10000000000000010                                                                              0000000000000001                                               10000000000000100                                                                              0000000000000011                                               10000000000001000                                                                              0000000000000111                                               10000000000010000                                                                              0000000000001111                                               10000000000100000                                                                              0000000000011111                                               10000000001000000                                                                              0000000000111111                                               10000000010000000                                                                              0000000001111111                                               10000000100000000                                                                              0000000011111111                                               10000001000000000                                                                              0000000111111111                                               10000010000000000                                                                              0000001111111111                                               10000100000000000                                                                              0000011111111111                                               10001000000000000                                                                              0001111111111111                                               10010000000000000                                                                              0011111111111111                                               10100000000000000                                                                              0111111111111111                                               11000000000000000                                                                              0111111111111111                                               ______________________________________                                    

The operation of the operation circuit according to the second circuit will be described with reference also to FIG. 1, using the parenthesized numerals. The input signal 2a having the information regarding the position at the rounded bit of the rounded input signal 1 is entered to the decoder circuit 3a, which decodes the input signals 2a and 5 according to the truth table 4 to generate the output signal 4. The output signal 4 and the input signal 1 are added by means of the arithmetic logic means 6 and the result is emitted as the output signal 7. The bits less significant than the rounded bits according to the input signal 2a are all turned into zero by the rounding-down circuit 8 and the result is emitted as the output signal 9. Incidentally, the truth table of the decoder circuit 11a is illustrated in the following table 5.

                  TABLE 5                                                          ______________________________________                                         Second Input Signal                                                                            Decoder Output                                                 ______________________________________                                         0000000000000001                                                                               1111111111111110                                               0000000000000010                                                                               1111111111111100                                               0000000000000100                                                                               1111111111111000                                               0000000000001000                                                                               1111111111110000                                               0000000000010000                                                                               1111111111100000                                               0000000000100000                                                                               1111111111000000                                               0000000001000000                                                                               1111111110000000                                               0000000010000000                                                                               1111111100000000                                               0000000100000000                                                                               1111111000000000                                               0000001000000000                                                                               1111110000000000                                               0000010000000000                                                                               1111100000000000                                               0000100000000000                                                                               1111000000000000                                               0001000000000000                                                                               1110000000000000                                               0010000000000000                                                                               1100000000000000                                               0100000000000000                                                                               1000000000000000                                               1000000000000000                                                                               0000000000000000                                               ______________________________________                                     

What is claimed is:
 1. A rounding operation circuit for performing a rounding operation on a first input signal represented in a two's complement form, comprising decoder means for receiving a second input signal designating a rounding position of said first input signal and for outputting a decoded signal in which a bit corresponding to said rounding position takes logic "1" and remaining bits take logic "0" when said first input signal is positive and in which a bit or bits less significant that said bit corresponding to said rounding position takes logic "1" and remaining bit or bits take logic "0" when said first input signal is negative; operational means for performing an adding operation on said first input signal and said decoded signal and for outputting a resultant signal; and rounding-down means performing such a rounding-down operation on said resultant signal that bits of said resultant signal less significant than said rounding position take logic "0".
 2. The circuit according to claim 1 wherein said second input signal for specifying the rounding position of said first input signal comprises four bits.
 3. The circuit according to claim 1 wherein said second input signal for specifying the rounding position of said first input signal is comprised of 16 bits in which the value specifying the rounding position has an immediate value.
 4. A rounding circuit for rounding input data (1) represented in a two's complement form to produce rounded data, comprising a decoder supplied with control data indicative of a rounding position of said input data and outputting first decoded data when said input data is positive and second decoded data when said input data is negative, said first decoded data consisting of a bit that takes a first logic level and corresponds to said rounding position and remaining bits that take a second logic level, said second decoded data consisting of a bit or bits that take said first logic level and are less significant than said bit corresponding to said rounding position and remaining bit or bits that take said second logic level, and operational unit adding said input data with the decoded data derived from said decoder and outputting added data, and a rounding-down circuit producing said rounded data to said added data and said control data, said rounded data having a bit or bits that are less significant than a bit corresponding to said rounding position and rounded down to said second logic level. 